Exhaust system

ABSTRACT

An exhaust system or an engine ( 12 ) includes a lean NOx catalytic device ( 18 ), and a heat exchanger ( 70 ) positioned upstream of the catalytic device ( 18 ). Control means ( 44, 46 ) controls a valve ( 36 ) to regulate exhaust gas flow through the heat exchanger ( 70 ) or along a bypass path ( 26 ). The heat exchanger ( 70 ) can cool the exhaust gases to ensure that the maximum operating temperature of the catalytic device ( 1 ) is not exceeded. During use, the heat exchanger ( 70 ) can be bypassed to allow high temperature purge cycles.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national application of internationalapplication serial No. PCT/GB99/01560 filed May 17, 1999, which claimspriority to Great Britain Patent Application No. 9810601.6 filed May 15,1998.

The present invention relates to an exhaust system for an internalcombustion engine, in particular to an exhaust system employing acatalytic device for purifying the exhaust gases. The invention isespecially suitable for a system for a lean burn engine (employing alean NOx catalytic device), but it is not limited exclusively to this.

In general terms, the need to operate a catalytic device above a minimumoperating temperature is well known in the art. For example,EP-A-0460507, GB-A2278068 and WO 96/27734 describe arrangements forrouting the exhaust along an appropriate exhaust path if the gas is notat an optimum high temperature, or if the catalytic devices have not yetreached there optimum temperatures.

The increasing cost of fuel and the concern over CO₂ emissions has leada drive for engines with improved fuel economy. Lean burn engines havebeen developed using gasoline direct injection and port injectiontechniques.

Under these lean operating conditions the standard 3-way catalyst isvery efficient for CO and hydrocarbon (HC) oxidation, but the reductionof oxides of nitrogen NOx (NO and NO₂) to di-nitrogen (N₂) isconsiderably more difficult. Catalytic converters and traps are beingdeveloped which can operate under lean conditions. The “lean” problem isthat, there is generally an insufficient quantity of hydrocarbons in theexhaust gas to enable efficient conversion of all of the NOx todi-nitrogen at the catalytic device. One type lean burn engine uses alean cycle and an intermittent stoichiometric or rich cycle. A catalytictrap can be used which absorbs the excess NOx gases during the leancycle, and then converts the NOx to N₂ in the presence of morehydrocarbons during the rich cycle. The rich cycle is sometimes referredto as the “purge” cycle.

Although lean NOx catalytic converters and traps offer potentiallyenormous emissions benefits, it has been extremely difficult to attainthe full potential of the catalytic devices, especially under conditionsin which the engine is working hard (for example, for high speed vehiclecruising). The reason is that, under such conditions, the temperature ofthe exhaust gas entering the catalytic trap often exceeds the optimumoperating range for the catalytic device. For example, FIG. 17illustrates the typical temperatures characteristics for a lean NOxtrap. The catalytic material has a coating for absorbing the excess NOx,but this is only effective up to about 450° C. On the other hand, thereduction of the oxides in the presence of hydrocarbons is onlyeffective at temperatures above about 200° C. This creates a usefultemperature window from approximately 200-450° C. in which the lean NOxconversion can occur. At temperatures outside this window (for example,caused by high engine speed), the catalytic trap will not operateefficiently. Lean NOx catalytic converters also operate in a similartemperature range.

Broadly speaking, one aspect of the present invention is to provide acooling heat exchanger unit upstream of a catalytic device, and acontrol device for providing selective cooling of the exhaust gasupstream of the catalytic device, using the heat exchanger.

With the invention, the heat exchanger unit can provide sufficientcooling to cool the hot exhaust gases to a desired catalytic operatingtemperature, or to within a desired operating temperature window, forefficient catalytic operation.

Moreover, cooling of the exhaust gases provides other performanceadvantages, specifically by reducing the volume of the gas, and thus thevolume flow rate through the exhaust system. This can help reduce thebackpressure within the exhaust system, and can also help reduce flownoise through the system, especially at high engine speeds and loads.These are significant problems associated with lean NOx catalyticdevices, which tend to require relatively large substrates for efficientlean NOx operation. The use of large substrates can cause undesirablebackpressure build up. The reduction in back pressure will help toimprove fuel economy and reduce CO₂ emissions.

The heat exchanger unit may be a gas cooled unit (for example, aircooled), or it may be liquid cooled. The latter is preferred for thefollowing reasons:

(a) A liquid-cooled heat exchanger can avoid the occurrence of transienttemperature drops which air-cooled exchangers can cause. Initially, anair-cooled heat exchanger will be much colder than the hot exhaust gasesand, when the hot gases first pass through the exchanger, the largetemperature difference causes a very efficient heatsink effect to occur.Such large transients can cause the temperature to fall below an optimumoperating range of the catalytic device until the heat exchanger heatsup to near the exhaust gas temperature;

(b) A liquid-cooled heat exchanger remains at the temperature of thecoolant, and never heats up to the exhaust gas temperature. Heattransfer is achieved through the large heat capacity of the liquid, anddoes not depend (at least to much extent) on the precise temperature ofthe coolant itself. In contrast, an air-cooled exchanger necessarilyheats up to near the exhaust gas temperature, and dissipates heat bybeing much hotter than the surroundings. This can cause design problemsfor placement on a vehicle away from hazardous (temperature sensitive)areas, and also requires the presence of a cooling air flow, in use.

(c) A liquid-cooled heat exchanger can enable the use of an open-loopcontrol system for controlling the cooling operation without having tomeasure directly the temperature of the exhaust gas in the exhaustsystem. Most vehicles are not equipped with an exhaust temperaturesensor, and the addition of such a sensor able to withstand harshexhaust conditions represents additional expense. With a liquid-cooledsystem, the exhaust gas temperature can be predicted using the outputsfrom conventional vehicle sensors for sensing, for example, the engineinlet air temperature, the engine coolant temperature, the engine speed,the air mass flow entering the engine, and the fuel:air mixture(measured using a lambda sensor).

(d) A liquid heat exchanger can generally be made more compact than aair-cooled heat exchanger.

If a liquid heat exchanger is used, then preferably, this is coupled toan existing coolant circuit of a vehicle, such as, for example, theengine coolant circuit.

If a gas-cooled heat exchanger is used, then the arrangement shouldcomprise a gas inlet tube, a heat exchanger unit coupled to the inlettube, and an outlet tube exiting the heat exchanger unit, the heatexchanger unit having a greater heat dissipation effect than the inletand outlet tubes.

In either type of system, the exhaust system preferably comprises afirst flow path through the heat exchanger for cooling the gas in thefirst path, and a second flow path bypassing the heat exchanger. Thesecond path may flow through the housing of the heat exchanger along asubstantially non-heat exchange (or at least a low-heat exchange) path.

In another broad aspect, the invention provides a method, and also acontrol apparatus, for controlling operation of a cooling device forcooling exhaust gas upstream of a catalytic exhaust purification device.

In one preferred aspect, the method includes predicting the exhaust gastemperature from a plurality of characteristics which are each notdirectly indicative of the exhaust temperature, and controlling coolingoperation in response to the predicted exhaust gas temperature.

In another preferred aspect, the method includes controlling the coolingduring a first engine cycle to achieve an exhaust temperature within afirst operating range for the catalytic device, and during a secondengine cycle to achieve an exhaust temperature within a second operatingrange for the catalytic device.

The second operating range (achieved after the first operating range)may include a higher maximum temperature than the first operating range.For example, the second operating range may correspond to astoichiometric cycle, or to a sulphur purge cycle. The first cycle maycorrespond to a lean cycle.

Embodiments of the invention are now described by way of example only,with reference to accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a first embodiment of an exhaustsystem for a lean bum engine;

FIG. 2 is a schematic view illustrating the heat exchanger in moredetail;

FIG. 3 is a schematic view illustrating a comparative prior art exhaustsystem;

FIG. 4 is a graph illustrating gas temperatures during steady statecruising;

FIG. 5 is a graph illustrating the improvement in catalytic conversionefficiency;

FIG. 6 is a graph illustrating the behaviour of the system of FIG. 1during a drive cycle;

FIG. 7 is a graph comparing engine torque and power in the heatexchanger valve-open and valve-closed positions;

FIG. 8 is a schematic view illustrating a second embodiment of exhaustsystem;

FIG. 9 is a schematic section through the heat exchanger used in FIG. 8;

FIG. 10 is a plan view in isolation of a baffle for the heat exchangerof FIG. 9;

FIG. 11 is a plan view in isolation of an end plate of the heatexchanger of FIG.9;

FIG. 12 is a graph illustrating the performance of the secondembodiment;

FIG. 13 is a more detailed view of a portion of FIG. 12 illustrating theeffect of coolant temperature;

FIG. 14 is a schematic section through an alternative design of heatexchanger usable in the embodiment of FIG. 7;

FIG. 15 is a flow diagram illustrating the steps used to controloperation of the exhaust system;

FIG. 16 is a schematic diagram illustrating a control algorithm; and

FIG. 17 illustrates conversion efficiency of a conventional lean NOxcatalytic trap.

Referring to FIG. 1, a test exhaust system 10 is illustrated for a leanburn engine, identified schematically at 12. The exhaust systemcomprises an exhaust manifold 14 coupled to the exhaust ports of theengine 12, a conventional light-off catalytic converter 16 arrangedclose to the engine 12 to provide catalytic purification when the engineis first run, and a lean NOx catalytic device 18 arranged downstream ofthe light-off converter 16. The lean NOx device 18 may either be acatalytic trap, or a lean catalytic converter, to suit the engine 12.

Arranged between the light-off converter 16 and the lean NOx device 18is a cooling arrangement 20 which consists of a heat exchanger unit 22arranged in a first gas flow path 24, and a second gas flow path 26bypassing the heat exchanger unit 22.

Referring to FIG. 2, in this embodiment the heat exchanger unit 22 isair cooled, and comprises a linear radiator arrangement of nine steelexchanger tubes 28 extending between an inlet manifold tube 30 and anoutlet manifold tube 32. The exchanger tubes 28 are cooled by movingair, represented by the fan 34 (FIG. 1).

In the illustrated test arrangement, the exchanger tubes 28 areapproximately 600 mm long, with an inside diameter of about 22 mm. Thefan 34 provides an ambient air speed of about 2.5 m/s over the heatexchanger unit 22.

Flow through the first and second paths 24 and 26 is controlled by avalve 36 situated in the first flow path 24 downstream of the heatexchanger unit 22. The flow resistance of the second path 26 relative tothe first path 24 is such that, when the valve 36 is open, a substantialportion of the gas flows through the first path 24 through the heatexchanger 22. When the valve 36 is closed, the gas has to flow throughthe second path 26, and thereby bypasses the heat exchanger 22. The flowrates through the first and second paths are selected such that neitherpath presents too high an impedance, which would otherwise causeundesirable back pressure in the exhaust path.

In the test arrangement illustrated in FIG. 2, the impedance of thesecond path 26 is made adjustable by means of a replaceable constrictionassembly 38. The assembly 38 consists of two flanges 40 between which isreceived an exchangeable disc 42 having an orifice of a predeterminedsize.

The valve 36 is a vacuum controlled butterfly valve, which is controlledby means of an electrical solenoid 44. The solenoid is controlled by acontrol unit 46, described further below.

The above valve control arrangement is preferred, as it avoids the needto place a valve in the direct flow of very hot exhaust gases. Instead,the valve 36 is placed downstream of the heat exchanger unit, and so isexposed to less hot exhaust gas. This can increase valve life, andenable a less expensive valve to be used. However, it will beappreciated that in other embodiments, a flow switching valve may beused in the second flow path 26 if desired, or at one of the junctionsbetween the first and second flow paths 24 and 26 if desired. The valvemay be a butterfly type or other type of valve, as appropriate.

In this embodiment, a temperature sensor 48 measures the exhaust gastemperature upstream of the lean NOx catalytic device 18. For example,the temperature sensor 48 may be located at the inlet to the device 18,or upstream of the heat exchanger unit 22. The control unit 46 may, forexample, be a straightforward threshold sensing unit (with hysteresis ifdesired) which controls the valve 36 to open when the exhaust gasexceeds a threshold temperature, so that the temperature is maintainedin a desired temperature window. Alternatively, the control unit 46 mayinclude a predictive control algorithm representing a thermal model ofthe exhaust system to predict the exhaust gas temperature depending onthe load conditions of the engine.

To test the effect of the heat exchanger, the same exhaust system wasalso used in a conventional test arrangement, as illustrated in FIG. 3.Referring to FIG. 3, features described above are denoted by the samereference numerals, where appropriate. In this conventional testarrangement, the heat exchanger of FIG. 1 is replaced by a steel tubeapproximately 750 mm long. This is equivalent to the path length theexhaust gas travels when the valve 36 of FIG. 1 is closed. This pipelength is also representative of the typical distance between a closecoupled (light-off) catalytic converter in a vehicle engine bay, and anNOx trap in an underfloor position on a vehicle.

FIGS. 4, 5 and 6 illustrate the performance comparisons between thearrangements of FIGS. 1 and 3. The engine used was a 1.8 litrefour-cylinder homogeneous lean burn engine coupled to a 100 KW DCdynamometer, to simulate appropriate loading on the engine.

FIG. 4 illustrates the exhaust gas temperature at the inlet of the leanNOx catalytic device 18 at an engine speed and load corresponding tovehicle cruising at a speed of 120 Km/h (about 75 mph). Bar 50represents the temperature for the conventional system of FIG. 3,reaching about 600° C., which is well outside the operating window of200-450° C. for the catalytic device 18. With the heat exchanger unit 22in place, and the control valve 36 open, the temperature is reduced toabout 424° C. as illustrated by bar 52, which is inside the optimumtemperature range.

FIG. 5 illustrates the NOx conversion efficiency of the lean NOx device18 for the above conditions. For an exhaust gas temperature of about600° C., bar 54 shows that the conversion efficiency is less than 10%,resulting in high NOx pollution. However, for the lower exhaust gastemperature achieved with the heat exchanger unit 22, bar 56 shows thatthe conversion efficiency approaches 50%.

FIG. 6 illustrates the exhaust gas temperature (at the inlet to the leanNOx catalytic device 18) over the first 1200 seconds of the standardreference European drive cycle. Line 58 illustrates the temperature forthe conventional exhaust arrangement of FIG. 3. In the urban drive cycle(portion 60), the temperature reaches the minimum operating temperatureof 200° C. for the lean NOx catalytic device 18 after about 150 seconds.The temperature remains below the maximum threshold of 450° C.throughout the urban portion of the drive cycle (portion 60). However,during the extra urban portion (portion 62), the temperature quicklyexceeds the maximum threshold of 450° C.

Line 64 illustrates the catalytic device inlet temperature for theexhaust arrangement of FIG. 1. In the urban drive cycle portion 60, thetemperature reaches the minimum lean NOx catalytic operating temperatureafter about 250 seconds, the gas exhaust temperature being about 50° C.below that with the exchanger unit 22 removed, even though during thisportion of the cycle the valve 36 is closed. This temperature reductionis believed to be a result of direct heat conduction through the metaltubes of the exhaust system, resulting in some heat loss through theheat exchanger unit 22. In the extra urban portion 62 of the cycle, thetemperature begins to rise, resulting in the valve 36 opening to allowgas through the heat exchanger unit 22. The temperature falls abruptly,and remains below the 450° C. threshold.

As described previously, the gas flow rates through the first and secondflow paths 24 and 26 (FIGS. 1 and 2) are designed such that the flowdistribution can be controlled by a single valve 36 downstream of theheat exchanger unit 22. FIG. 7. illustrates a comparison of the enginepower and torque curves for the open and closed conditions of the valve36. Any large variation in engine performance would be very undesirable,as this would affect the drivability of the vehicle, depending onwhether the valve were to be open or closed. However, as can be seen,there is very little change in the engine performance when the valve isswitched.

It will be appreciated that the cooling arrangement illustrated abovecan provide significantly better NOx conversion performance compared toa conventional exhaust arrangement. The use selective cooling (providedabove by two flow paths) can ensure that cooling is only used whenneeded, i.e. when the exhaust gas temperature becomes elevated. Duringinitial running of the engine (and during NOx purge and sulphur purgecycles), the cooling can be bypassed, to ensure that the lean NOxcatalytic device 18 reaches the desired operating temperature, or purgetemperature, quickly.

A further and important benefit in cooling the exhaust gases is that itinherently reduces the volume of the gas, and the thus the volume flowrate of the gas through the exhaust system. Such a reduction can reduceback-pressure and also the flow noise in the exhaust system.Back-pressure in a lean NOx system is a very important consideration,because the catalytic substrates used for the lean NOx catalytic devicesgenerally have to be relatively large to provide good performance inlean conditions. Such large substrates can result in a back-pressureincrease, and so any means of reducing the back-pressure is highlydesirable.

One of the features of the air-cooled heat exchanger system describedabove is that there tends to be a large transient temperature drop whenthe control valve 36 is switched to the open condition. Such a transientdrop is visible in FIG. 6 at point 66. This is a result of the heatexchanger unit 22 being initially very cool (since it is cooled by thefan 34), and acting as a very efficient heatsink when the exhaust gas isfirst passed through the heat exchanger 22. As more exhaust gas passesthrough the heat exchanger 22, the heat exchange tubes 28 heat up, andprovide a lesser rate (by dissipating the heat in the air streamprovided by the fan 34). Such a transient may be undesirable, as it cancause the exhaust gas temperature to fall below the minimum activationtemperature for the lean NOx catalytic device 18 (about 200° C.), forexample as illustrated by the point 66 in FIG. 6.

FIGS. 8-11 illustrate a second embodiment, which can provide all of theadvantages of the first embodiment, and also addresses the transientproblem. Where appropriate, the same reference numerals have been usedto denote features equivalent to those described previously.

The principle difference in FIG. 8 is the use of a liquid-cooled heatexchanger unit 70 in place of the air-cooled heat exchanger unit 22 ofFIG. 1. The liquid-cooled heat exchanger 70 consists generally of ahollow housing 72 which, in this embodiment, is cylindrical and containsan arrangement of gas carrying tubes 74 arranged as a uniform “bundle”,with spacing between adjacent tubes to allow thermal contact with thesurrounding coolant liquid. The tubes 74 extend between two end plates76 which are apertured to define an openings 77 into which each tube 74opens at its end. The ends of the tubes 74 are welded to the end platesin a liquid-tight manner. Outside the end plates 76, the housing definesan inlet chamber 78 to allow the incoming exhaust gas to be distributedto flow into the tubes 74, and an outlet chamber 80 for there-collimation of the gas flowing out of the tubes 74.

The housing 72 defines a liquid-tight chamber surrounding the tubes 74.Liquid coolant is received through a coolant inlet port 82 and iscirculated in the housing before exiting through a coolant outlet port84. In order to ensure optimum flow of the coolant in contact with thetubes 74, the housing includes a plurality of internal baffles 86. Eachbaffle is similar to the end plates 76 in that it consists of a wallwith openings 88 through which the tubes 74 pass. However, each baffleincludes a “cut-away” portion to define a passage between the edge ofthe baffle and the housing to permit the flow of liquid around thebaffle. As best seen in FIG. 9, the baffles 86 are arranged alternatelyto define a tortuous sinusoidal flow path for the coolant liquid betweenthe inlet and outlet ports 82 and 84.

In the present embodiment, the heat exchanger 70 is made of steel, andis relatively compact, including 19 tubes 74 each of length 440 mm anddiameter 14 mm. The housing has a diameter of about 88 mm, and thebaffles each have a “height” of about 60 mm. The baffles are arrangedwith a uniform spacing of about 110 mm, and are secured in position bybeing spot welded to, for example, three of the tubes 74.

Liquid coolant circulated through the heat exchanger 70 by a liquidcoolant circuit 90 which includes a heat dissipating radiator 92 and acoolant pump 94. The coolant circuit may be a dedicated circuit in thevehicle, but in this preferred embodiment, the coolant circuit is partof an existing coolant circuit on the vehicle, for example, the usualengine coolant circuit and using the engine radiator (92) and the enginecoolant pump (94). This can avoid the additional space and cost of usingan independent cooling circuit.

FIG. 12 illustrates the performance of the exhaust system with theliquid-cooled heat exchanger, and using a similar engine and testarrangement as that described previously. In FIG. 12:

the line 96 represents the temperature of the exhaust gases at the inletto the heat exchanger (equivalent to the exhaust gas temperaturereaching the lean NOx catalytic device 18 if the heat exchanger were tobe omitted);

the line 98 represents the temperature of the exhaust gas leaving theheat exchanger (equivalent to the temperature of the exhaust gasentering the lean NOx device 18 when the control valve 36 is open);

the line 100 represents the temperature of the liquid coolant beingcirculated through the heat exchanger; and

the line 102 represents the mass flow of the exhaust gases.

The graph illustrates the measured characteristics over a cycleincluding three different engine settings, the first portion 104 beingat an engine speed of 1000 rpm at 10% throttle, the second portion 106being at an engine speed of 2000 rpm at 50% throttle, and the thirdportion 108 being at an engine speed of 4000 rpm at 100% throttle.

As can be seen from the graph, the relatively compact heat exchangerprovides adequate cooling to maintain the exhaust gas temperature belowabout 450° C. even at elevated inlet temperatures, and high mass flow.

Moreover, the liquid heat exchanger does not produce any transients whenthe flow of the exhaust gas is switched from the bypass path to the heatexchanger path. This is because, unlike air-cooling, the walltemperature does not vary much. Rather, it is the high specific heatcapacity of the coolant liquid which enables heat to be absorbed by thecoolant, with little resultant temperature dependency. For example,referring to FIGS. 12 and 13, in the portion 106 of the test cycledescribed above, the water temperature in the heat exchanger fluctuatesbetween about 80° C. and 90° C. However, there is virtually no resultantchange in the gas outlet temperature from the heat exchanger (line 98).

A further advantage with a liquid coolant heat exchanger is that, incontrast to an air-cooled exchanger, the exchanger does not heat up tothe high exhaust gas temperatures. The exchanger remains at thetemperature of the coolant. This can avoid the need to provide hightemperature dissipation devices in the exhaust system, which might provehazardous or position critical for underfloor exhaust systems, or forengine-bay exhaust components. The lack of any requirement for a coolingair flow over the exchanger also permits the designer greaterflexibility in positioning the exchanger on a vehicle.

FIG. 14 illustrates an alternative design of liquid coolant heatexchanger 110, which incorporates the bypass, non-heat exchange path,within the housing 112 of the heat exchanger 110. This avoids the needto employ separate conduits for the exhaust bypass path. Referring toFIG. 14, the housing 112 through which the coolant flows has a generallyannular shape, and the heat exchange tubes 74 are arranged in an annularconfiguration within the housing 112. The central hollow of the housingprovides the bypass path 26 with little, or no, thermal contact with thecoolant medium. The heat exchange and non-heat exchange paths join ateither end of the housing 112 at an inlet chamber 114 and an outletchamber 116. The valve 36 is arranged within the bypass path and, inthis embodiment, can be an integral part of the heat exchanger unit.

If desired, it is possible to concatenate the above heat exchanger 110with a catalytic device within a common housing, to provide a singleunit which contains a catalytic device and a temperature regulatingmechanism.

FIG. 15 illustrates a typical control process loop 120 for controllingthe valve 36 during the lean, rich and sulphur purge cycles of theengine. Step 122 determines whether the engine is running and, if not,the process branches to a termination step 124.

If the engine is running, step 126 determines whether a sulphur purge isnecessary to clear the exhaust system of a build up of sulphur oxides.In some countries, fuel contains a fairly high sulphur content, and thesulphur oxides tend to collect in the catalytic devices (and act incompetition to the conversion of nitrogen oxides). The build up ofsulphur oxides is countered by a high temperature purge. If a sulphurpurge is necessary, then step 126 branches to step 128 at which a targettemperature window defined by Tmax, Tmin is set to correspond to thedesired high temperature for a sulphur purge, generally between about600° C. and 750° C. Step 130 controls the valve 36 to try to achieve atemperature within the window. Generally, the desired temperature is sohigh that the valve 36 will remain closed during this period to allowthe exhaust temperature to reach maximum levels.

Step 132 determines whether the sulphur purge has been completed. Ifnot, the process loops back to repeat steps 128 and 132 until completionof the sulphur purge.

Once the sulphur purge has been completed, or if no sulphur purge wasdetermined to be necessary at step 126, the process proceeds to step 134which determines whether the engine is currently running lean. If theengine is running lean, then the process proceeds through step 136 atwhich a target temperature window defined by Tmax, Tmin is set tocorrespond to the temperature range for lean NOx catalytic operation,generally between about 200° C. and 450° C. If the engine is not runninglean, then the target temperature window is set at step 138 tocorrespond to stoichiometric NOx catalytic operation, generally between350° C. and 750° C.

The process then proceeds to step 137 which controls the valve 36 to tryto achieve a temperature within the target window. Thereafter, theprocess loops back to step 122 described above.

The valve 36 may be controlled either to be fully open of fully closed.Alternatively, the valve 36 may be controlled to be open by acontrollable amount, through the use of proportion control, for examplePID (proportional integral differential) control.

The valve 36 may be controlled simply through the use of a temperaturesensor which measures directly the temperature of the gas in the exhaustsystem (closed loop feedback). However, the use of a liquid cooled heatexchanger system also permits an open loop control to be used whichpredicts the temperature of the exhaust gas without having to measurethe exhaust temperature directly. This can provide cost savings in nothaving to use a relatively expensive exhaust gas temperature sensor.

An open loop system is illustrated, for example, in FIG. 16. The systemuses the outputs of sensors which are provided as standard sensors onmost modem vehicles. These are: an air temperature sensor 140 whichprovides a signal indicative of the inlet air temperature to the engine;a coolant temperature sensor 142 which provides a signal indicative ofthe engine coolant temperature; an engine speed sensor 144 whichprovides an indication of the rpm engine speed (as measured or asdeduced from the engine control system); and air mass flow sensor 146which provides a signals indicative of the air mass flow into theengine; and a lambda sensor 148 which provides a signal indicative ofthe air:fuel ratio as measured from the exhaust gases.

An engine map/model 150 is used to calculate the exhaust gas temperatureand the exhaust gas mass flow from the engine, and an exhaust systemthermal model 152 is then used to calculate the amount of coolingrequired to bring the exhaust gas temperature to within the targettemperature window, based on the liquid coolant temperature (forexample, the same as the engine coolant temperature if a common system).

The engine map/model 150, and the thermal model 152 of the exhaustsystem (including the heat exchanger), can be implemented relativelyeasily using a computer based control system, for example, a microcontroller.

It will be appreciated that the invention, particularly as described inthe preferred embodiments, can provide a system for controlling thetemperature of exhaust gases to within the desired operating temperaturewindow for a catalytic device.

It will be appreciated that the above description is merely illustrativeof preferred embodiments of the invention, and that many modificationsmay be made within the scope of the invention. Features believed to beof particular importance are defined in the appended claims. However,the Applicant claims protection for any novel feature or aspectdescribed herein and/or illustrated in the drawings, whether or notemphasis has been placed thereon.

What is claimed is:
 1. An exhaust system for an internal combustionengine defining a gas flow path and comprising a NOx catalytic devicefor purifying the exhaust gases, a liquid cooled heat exchanger upstreamof the NOx catalytic device, and a flow control valve, the gas flow pathincluding a cooling gas flow path through the heat exchanger and abypass gas flow path, the flow control valve adapted to selectivelyroute the flow of gas through the cooling gas flow path for cooling andthe bypass gas flow path so that the gas entering the catalytic deviceis cooled to a desired catalytic operating temperature whereby NOxconversion efficiency is maintained.
 2. A system according to claim 1wherein the flow control valve has an open position for routing the flowof gas substantially through the cooling gas flow path and a closedposition for routing the flow of gas substantially through the bypassgas flow path.
 3. A system according to claim 1 wherein the NOxcatalytic device comprises a catalytic trap.
 4. A system according toclaim 1 wherein the bypass gas flow path is substantially outside of theheat exchanger.
 5. A system according to claim 1 wherein the bypass gasflow path passes through the heat exchanger.
 6. A system according toclaim 5 wherein the heat exchanger defines the cooling gas flow path andthe bypass gas flow path and includes a coolant medium, the coolantmedium having substantially more thermal contact with gas in the coolinggas flow path than with gas in the bypass gas flow path.
 7. A systemaccording to claim 6 wherein the flow control valve is located in thecooling gas flow path, the cooling gas flow path having a flowresistance not significantly less than the bypath gas flow path topromote gas flow through the cooling gas flow path when the controlvalve is open.
 8. The system according to claim 1 further comprising asecond catalytic device upstream of the heat exchanger.
 9. The systemaccording to claim 8 wherein the second heat catalytic device comprisesa light-off catalytic converter.
 10. The system according to claim 1wherein the heat exchanger comprises a coolant inlet port and a coolantoutlet port for enabling coolant to be circulated through the heatexchanger unit.
 11. The system according to claim 10 further comprisinga coolant circuit coupled to the inlet and outlet ports, and a radiatorfor cooling the coolant in the circuit.
 12. A system according to claim1 wherein the NOx catalytic device comprises an electronic circuitoperable to control the cooling of the exhaust gases in dependence on atleast one of the characteristic indicative of the exhaust gastemperature.
 13. A system according to claim 12 further comprising atleast one sensor for measuring said at least one characteristic and forproducing electronic signals representative of said measured at leastone characteristic.